This invention relates to shielding superconducting solenoids.
In some applications of superconducting solenoids, such as NMR, MRI, and mass spectroscopy of particles in an ion trap, variations in the high (e.g., 6 Tesla) magnetic fields produced within the solenoid must be limited to no more than, for example, 6 nanoTeslas (nT) per hour. This degree of stability is hard to achieve because of the influence of relatively greater fluctuations (e.g., between 10 nT and 10 .mu.T per hour) of the ambient magnetic field produced by, for example, ionospheric conditions, solar activity, and elevators or subways.
Shielding a high magnetic field solenoid from the ambient magnetic field is impractical using highly permeable metals, like iron, (because they become saturated by the high field), or using type I superconductors, such as lead or niobium (because the high field exceeds the critical field for such materials).
In A. Dutta et al., Rev. Sci. Instruments, volume 58, page 628 (1987), a cylinder of type II superconductor was used to screen ambient magnetic field fluctuations from a very small sized high magnetic field region.
R. S. Van Dyck, Jr. et al., Rev. Sci. Instruments, volume 57, page 593 (1986), describe using two serially connected, concentric, coplanar superconducting loops to impose a tunable gradient on a high magnetic field. The radii of the loops were chosen to minimize the shift of the high magnetic field at the center as the gradient was tuned; Ambient magnetic field fluctuations were expected to be reduced by a factor of ten at the center.